Is there a way to render single frame using network rendering in Cycles?
I'm not sure if a render farm currently exists to do this, but theoretically it's easy. Simply use a method called Image Stacking, where you render the same image multiple times with different seeds and then mix them together to average out the noise.

The first two images would mix 50/50, the third would mix into that by 33.3%, the fourth 25%, 5th 20%... basically 1/n where n is the number of images in the mix.

This is the method that renderfarm.fi uses, but I believe they have a minimum animation length so you can't use it to render a single still image.